The candidate devices presently available for obtaining mechanical advantage in rotary positioning mechanisms are: harmonic drives, planetary gear systems and worm gear reducers. Each of these devices have characteristics which limit their usefulness as precision positioning mechanisms. Harmonic drives have very high reduction ratios and low stiffness. Thus, very high input speeds are required to generate reasonable output speeds, and low system natural frequencies present potential design problems. Planetary gear systems have inherent backlash which is fundamentally incompatible with precision control. Precision worm gear reducers are expensive, bulky and have high levels of friction. Each of the cited shortcomings represents an effective limit on peak performance.
Accordingly, several objects of my invention are to provide a precision rotational positioning mechanism possessing mechanical advantage, which is essentially free of backlash and which posesses high rotational stiffness from input to output and low friction, i.e., the friction inherent in rolling contact bearings. Rotational stiffness from input to output is a measure of the amount of deformation of intermediate parts. High rotational stiffness permits the output to proportionally follow the input with minimum error. It also contributes to high system natural frequencies which permit accurate positioning to be performed at high rates of acceleration.
Another object of my invention is to provide a power transmission device with the previously described attributes, i.e., mechanical advantage, zero backlash, low friction and high rotational stiffness.
A further object of my invention is to permit the manufacture of the mechanism at low cost by using a standard tapered roller bearing as the key element in the mechanism.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description thereof.